Chemical vapor deposition (CVD) reactors are used to produce polycrystalline silicon (polysilicon), the key raw material used in silicon-based solar wafers and cells. Polysilicon is also used to make semiconductor wafers for microelectronic applications. The most widely used method for producing polysilicon is the Siemens reactor process, which has been in existence for about fifty years. In this process, high temperature polysilicon rods are placed in a reactor, and trichlorosilane (TCS) gas is passed over these rods. The silicon in the gas is deposited on the rods, and when the rods are grown large enough, they are removed from the reactor. The end product is in the form of polysilicon rods or chunks, which can be further processed into ingots, then sliced into wafers that are made into solar cells, for example.
The CVD-based Siemens process for manufacturing polysilicon produces a large amount of the byproduct silicon tetrachloride (STC). For example, a maximum of about 20 kg of STC is made as a byproduct for every kg of polysilicon produced. It is possible, however, to convert STC back to TCS by reacting STC with hydrogen in the gas phase at high temperature. The product TCS can then be recycled to the CVD reactor for production of more polysilicon. If STC could not be recycled, there would be a huge loss of the primary raw material TCS and a cost for disposal of the byproduct STC.
To efficiently react STC with hydrogen to form TCS, high reactant gas temperatures (e.g., greater than 900° C.) are required. Current commercial systems for conversion of STC to TCS use retrofitted Siemens CVD reactors with electrically heated graphite rods to heat the reactant gases. This equipment has a number of problems. For example, because CVD reactors have a high volume to heated rod surface area ratio, the local velocities and the heat transfer coefficients in the reactor are low. Thus, very high rod surface temperatures are required (e.g., temperatures greater than 1400° C.) to heat the reactant gas to sufficient temperature. Furthermore, the retrofitted CVD reactors have a large, heavy baseplate, which is expensive and makes it inconvenient to add heat exchanger equipment for recovery of heat.
Moreover, the heated graphite rods in a retrofitted CVD reactor require a large number of electrical connections. For example, the reactor may require two to four electrical connections per hairpin, which are all a potential source of rod failure and erroneous ground faults.
Furthermore, CVD reactors have a high radiation heat loss to the shell, wasting large amounts of energy. Current retrofitted CVD reactors use insulation to reduce the heat loss to the walls, and feature a primitive heat exchanger for heat recovery. The insulation is expensive because it must be made of materials that do not react in a high temperature environment, and it must fit around the outside of the heating rods. Cheaper insulating materials do not exhibit an adequate lifetime due to reaction with reactant gases at the high temperatures involved. The insulation itself will heat nearly to the temperature of the heating rods. With the use of insulation and a primitive heat exchanger, a retrofitted CVD reactor for conversion of STC to TCS requires energy of at least 1.5 KWhr per kilogram of TCS manufactured, which is quite high. Also, various key components of the converter have limited lifetimes and must be replaced at regular intervals—including the heating elements, the electrical connections, the insulation, and components of the heat exchanger.
Purpose-built (non-retrofitted) systems for conversion of STC to TCS have been proposed, which promise to be more energy efficient and cheaper to build than retrofitted CVD reactors. However, such purpose-built systems are not widely used, and are not yet commercially available. U.S. Pat. No. 7,442,824 describes a purpose-built STC to TCS converter with heating elements and a reactor wall that are coated with silicon carbide (SiC) to prevent contamination and degradation of these components in high temperature reaction environments. The converter employs graphite heating rods, as used in retrofitted CVD reactors.
Thus, there is a need for a more efficient STC to TCS converter suitable for commercial use.